A DISTRIBUTED REACTIVITY MODEL FOR SORPTION BY SAILS AND SEDIMENTS .3. EFFECTS OF DIAGENETIC PROCESSES ON SORPTION ENERGETICS

Isosteric heats of phenanthrene sorption were calculated from isotherms measured at different temperatures on three natural soil materials having organic carbon fractions subjected to varying degrees of diagenetic alteration. The isotherms were highly nonlinear, and the sorption process became less exothermic with increased solid-phase loading. These observations are consistent with a classical surface adsorption reaction. However, isosteric heat values calculated using a pure supercooled solute standard state (-16.3 to +25.5 kJ/mol) were less exothermic than expected for a surface adsorption process and least exothermic for the sorbent having the highest sorption capacity. A model capable of reconciling these seemingly divergent observations envisions natural organic matter as a macromolecule that is gradually transformed by diagenetic processes from a loose, amorphous structure (e.g., fulvic acid) to one that is increasingly condensed and ultimately highly microcrystalline in extremely old organic matter (e.g., anthracite coal). The model provides a consistent basis for explaining the observed isosteric heat values and changes in isotherm linearity and organic carbon-normalized sorption capacities with increased diagenesis.